Method of manufacturing electrical apparatus



SeP'f- 18, 1962 ..1.ANGE| Uccl,JR 3,054,672

METHOD OF MANUFACTURING ELECTRICAL APPARATUS Filed Jan. 14, 1957 United States Patent O This invention relates to improvements in the method of manufacturing cathode ray tubes and particularly to improvements in the method of manufacturing the screen structures of cathode ray tubes which are to be used as the image reproducing devices of color television receivers.

While not limited thereto the invention is particularly applicable to the manufacture f screen structures which are characterized by the presence of large numbers of minute, closely spaced phosphor elements, different ones of which are emissive of light of different primary colors in response to electron impingement. posed to manufacture such screen structures by photographic processes, the steps of atypical process being outlined below. In this process the first step involves the deposition of a uniform layer of photosensitive material upon the substrate upon which it is desired to form the screen structure and which may be the interior surface of the cathode ray tube faceplate. This layer of photosensitive material is then subjected to illumination in those regions in which it is desired to deposit phosphor emissive of light of one particular color, thereby rendering the photosensitive material insoluble in these regions. Following this exposure phosphor emissive of light of the particular color in question is applied to the entire layer of photosensitive material and the resultant sub-assembly is then washed so as to remove unexposed portions of photosensitive material together with the phosphor deposited on these unexposed portions, while leaving in place the exposed portions of photosensitive material together with the phosphor deposited on the latter portions. In the past it has been the practice to repeat the foregoing steps for each of the other phosphor materials which it was desired to deposit, varying, in each case, the locations of the exposed portions of photosensitive material, until phosphors emissive of light of all the desired colors had been deposited in their respective desired locations. The aforedescribed process is described more fully in the copending application of Paul D. Payne, Jr., Serial No. 376,345, led August 25, 1953, and assigned to the assignee of the present invention.

While the aforedescribed process has yielded screen structures of the kind under consideration which are satisfactory for most purposes there has been observed, in such screen structures, an appreciable tendency for the particles of a given phosphor material to appear not only in those locations in which it was desired to have them permanently deposited, but also in locations in which it was desired to deposit only particles of a different phosphor. As a result portions of the screen structure which 'ere intended to emit only light of one color also emitted light of one or more of the colors of light emitted by the subsequently deposited phosphors. This, in turn, caused some reduction in the capacity of the screen structure to produce a saturated image.

Investigation of this diiculty revealed that it was due to the fact that the formation of each set of phosphor elements after the rst involved the deposition of particles of the particular phosphor material over the entire surface of the previously formed portions of the screen structure, including those portions to which other phosphors had previously been caused to adhere owing to selective ex- It has been pro-.

3,554,672 Patented Sept. 18, i552 ice posure of the underlying photosensitive material. Apparently in some areas these other, previously deposited phosphors had not merely adhered to the surface of the exposed photosensitive material but had actually penetrated into its interior, thereby leaving the surface of the photosensitive material uncovered in these same areas. In these uncovered areas, particles of subsequently deposited, `different phosphor materials tended to adhere, With the aforedescribed deleterious results.

A number of possible solutions to this problem have been proposed and tested. For example, it has been thought that the extent to which the earlier-deposited phosphor particles are able to penetrate into underlying exposed photosensitive material could be reduced by increasing the degree of exposure of this material. Accordingly the time during which each layer of photosensitive material was subjected to illumination was substantially lengthened. This not only lengthened prohibitively the time consumed by the exposure process but even extremely prolonged exposure did not appear to reduce appreciably the ability of the phosphor particles to penetrate into the photosensitive material.

Since the photosensitive material is normally an organic substance susceptible of being removed by heating, consideration was also given to the possibility of heating the partially completed screen structure between successive applications of phosphor layers by the aforedescribed technique. This not only incr-eased prohibitively the number of steps involved in the manufacturing process, but was not successful in achieving the desired objective since even those portions of the screen assembly from which photosensitive material had thus been ostensibly removed, still exhibited the aforementioned tendency to retain other phosphor particles deposited thereon. While the reason for this residual retentivity is not clear it may be due to the fact that appreciable quantities of photosensitive material remain trapped between the phosphor particles, even during heating.

Accordingly it is a primary object of my invention to provide an improved method of manufacturing a screen structure for cathode ray tubes, this improved method being characterized by the deposition of different phosphors in close spatial proximity without noticeable overlapping `of particles of one phosphor onto regions occupied by particles of another phosphor.

It is another object `of my invention to provide an improved method of photographically depositing different phosphor materials in close spatial proximity to each other without appreciable contamination of one phosphor material by the other.

To achieve the aforestated objects, as well as others which will appear, a process is carried out which is, in most respects, similar to that which has been previously described as characteristic of the prior art. More particularly the :substrate upon which it is desired to form the screen structure, and which may be the interior surface of the tube faceplate, is first coated with a layer of unexposed photosensitive material. This material is then exposed to illumination in those regions in which it is desired to form phosphor elements emissive of light of one particular color, following which phosphor material emissive of light of that color is `deposited over the entire surface of the photosensitive layer, including its exposed and unexposed portions. Following this, the previously formed portions of the screen structure are washed so that there remain in place only the exposed portions of photosensitive material, together with the phosphor material deposited thereon.

Up to this point the method embodying my invention is identicafl With a method which has been used in the past. In the past there would then have been applied another o layer of photosensitive material, as a preliminary step in the formation of elements of a different phosphor material. In accordance with my invention, however, this deposition of additional photosensitive material is preceded by artreatment which affects selectively only those portions of the screen structure on which previously deposited and exposed photosentive material and phosphor have remained in place during washing. In particular this treatment renders these portions non-retentive of subsequently deposited phosphor particles. To this end the entire surface of the partially completed screen structure is coated with an aqueous' solution of a substance chosen from the group of alkali salts and hydroxides. Following the application of this solution the formation of the screen structure proceeds as in the prior art, namely with the deposition of a new layer of photosensitive material, the exposure of this layer in those regions in which it is desired to form phosphor elements emissive of light of a second color, the coating of the entire layer of photosensitive material with phosphor of this second color, and the washing of the resultant structure so as to remove unexposed photosensitive material and phosphor material deposited thereon, while leaving in place only the exposed portions of this photosensitive layer and the phosphor material of the second color which is deposited on these exposed portions. In accordance with my invention the application of the aforementioned solution of an alkali salt or hydroxide is then repeated, following which the third phosphor is deposited by the same process as the ylirst two phosphors.

Alternatively, the alkali salt or hydroxide solution need not be deposited separately but may be incorporated into the screen structure by using as avehicle for its deposition the same slurry in which the prior phosphor material is deposited. This eliminates the additional step which is otherwise required to practice my invention, and makes its use more attractive from a commercial point of view. p Y v It is not known withrcertainty Vwhy the presence Vof an aqueous solution of an alkali salt or hydroxide prevents retention of the additional phosphor material by the exposed photosensitive material remaining from the previous deposition of' other phosphor material withou-t interfering with the retention of phosphor material by those portions of the screen, structure on which no photosensitive material used in the previous deposition of other phosphor material remains. However it is believed that this may be due to the fact that alkali salts or hydroxides, when present in solutions of certain critical proportions, tend to wet both photosensitive materials and phosphors but not glass. As a result these alkali salts or hydroxides, when applied in an aqueous solution of the requisite concentration, will adhereto the previously deposited phosphors and to the photosensitive material which retains these phosphors in place during subsequent treatment of `the previously formed portions of the screen structure, but will not adhere to Vthose portions from which the previously deposited phosphor and photosensitive material has been washed olf. Since it is on these latter portions ofthe glass substrate that it is. desired to deposit other phosphorV materials the presence of the alkali salt or hydroxide-will not interfere with the deposition of these other phosphors in their desired positions but will prevent their deposition on previously formed` phosphor portions. In any event,l regardless of the theoretical explanation, experiment conclusively establishes that the aforedescribed results are indeed achieved. The important improvelments which my method produces will be better appreciated from a consideration of the following detailed discussion in conjunction with the accompanying drawings wherein: lFIGURE. 1 is an enlarged fragmentary view of a typical cathode ray tube screen structure for color television reproduction; and

FIGURE 2 is a flow diagram outlining a method of fabricating the screen of FIGURE l in accordance with my invention.

The cathode ray tube screen structure illustrated in FIGURE 1 of the drawings, to which more particular reference may now be had, is similar in form to certain prior art structures, consisting of a glass substrate 10 which is preferably the faceplate of the cathode ray tube itself but which may alternatively be a separate glass plate supported Within the tube envelope. Upon the electron beam confronting side of this glass substrate there are disposed a plurality of parallel phosphor strips, all those designated by reference numeral 11 being made of a fluorescent material emissive of red light in response to impingement by the electron beam of the cathode ray tube, all those designated by reference numeral 12 being made of fluorescent material responsive to electron beam impingement to emit green light and all those designated by numeral 13 being similarly responsive to emit blue light. Over the entire electron beam interceptive surface of this previously formed screen structure there is deposited a layer 14 of a conductive material, e.g. aluminum, of such thickness as to reflect light emitted from the phosphor strips while permitting electrons projected from the electron gun of the cathode ray tube to traverse this lm and activate the phosphor strips. On the surface of the conductive layer 14 which confronts the interior of the cathode ray tube there are further deposited a plurality of strips 15 which are disposed in a predetermined geometrical relationship to the phosphor strips and which are made of a material which responds to electron impingement in a substantially different manner than the aluminum film. For example, strips 15 may be made of magnesium oxide which has much higher secondary electron emissivity than aluminum and different ones of these strips 15 may be disposed in alignment with different green light emissive phosphor strips 12. These strips 15 then serve to produce distinctive electrical indications of electron beam impingement upon green light emissive phosphor strips. These indications may be used to facilitate the reproduction of a colored image upon the screen structure under consideration, but since the manner of such utilization forms no part of the present invention it is not further described.

A preferred manner of forming the phosphor strips 11, 12 and 13 in accordance with my invention is illustrated in the how chart of FIGURE 2, to which more parti-cular reference may now be had. The iirst step of this method, represented in FIGURE 2 by rectangle 16, involves the deposition of photosensitive material over the entire surface of the glass substrate upon which phosphor strips are to be deposited. This photosensitive material may consist, for example, of an aqueous solution of polyvinyl alcohol containing a suitable quantity of dichromate activator. The next step, which is represented in FIGURE 2 by rectangle 1'7, involves the exposure to illumination of only those portions of the photosensitive material deposited in step 16 in which it is desired to form, say phosphor strips 1l,'i.e. strips made of a phosphor'material emissive of red light. This may be accomplished by illuminating the desired portions through appropriately proportioned and aligned translucent strips in an optical mask disposed between the photosensitive layer and a suitable source of illumination. In the next step, represented in FIGURE 2 by rectangle 18, the previously deposited photosensitive material is coated with red light emissive phosphor material, this phosphor material being applied to all portionsof the photosensitive material, including the exposed and unexposed portions thereof. This is followed by a washing and partial drying operation, diagrarnmatically represented in FIGURE 2 by rectangle 19, in the course of which the previously formed portions of the screen structure are washed with distilled water with suiiicient vigor to remove substantially all of the photosensitive material which had not been exposed to illumination, and which had therefore not been rendered insoluble, together with the phospor deposited on these unexposed portions. This washing operation leaves in place, however, those portions of photosensitive material which have been exposed to illumination and the red phosphor deposited thereon.

The drying portion of this step of the operation may be carried out by simply exposing the interior of the bulb to the surrounding atmosphere. Alternatively the drying process may be accelerated by subjecting the screen structure to moderate heating from some heat source, such as an infra-red lamp, or by directing a stream of warm air onto its surface. This is intended to remove substantially all traces of the water introduced during the washing step, but drying is stopped before appreciable desiccation of the remaining photosensitive material takes place. Since, as has been pointed out, drying occurs inherently during the interval between Washing and the operation which follows drying, the drying step has been included with the washing step in the rectangle 19 of the flow chart of FIGURE 2.

In the next step of the method, represented diagrammatically in FIGURE 2 by rectangle 2i), there is applied a solution of a material selected from the group of alkaline salts and hydroxides, this solution being again applied to the entire interior surface of the previously formed portions of the screen structure, including particularly the phosphor coated portions and the exposed glass substrate between phosphor coated portions. Details of the quantities, concentrations and other parameters of this solution will be presented hereinafter.

As shown in the flow chart of FlGURE 2 by rectangle 21 this application of an alkali salt or hydroxide solution is followed by a deposition of additional photosensitive material, its exposure to illumination in those portions in which it is desired to form, say green light emissive phosphor strips 12, and its coating with green light emissive phosphor material. Each of the individual steps collectively represented by rectangle 21 may be identical to the corresponding one of steps 16 to 18 except that a different optical mask, or alternatively the same mask with its translucent portions displaced, is used for exposure and a diiferent phosphor material is applied to the photosensitive layer. The steps which are now collectively represented in the flow chart of FIG- URE 2 by rectangle 21 are followed by a repeated washing and partial drying operation represented by rectangle 22 and this, in turn, is `followed by repeated application of an alkali salt or hydroxide solution, represented by rectangle 23 in the flow chart. This second application of such a solution is again followed by the performance of steps similar to those represented by rectangles 16 to 18. These `final steps are collectively represented by rectangle 24 of FiGURE 2 and differ from steps 16 to 18 only in that the exposure of photosensitive material is made in those portions in which it is desired to deposit the remaining phosphor material strips, i.e. strips 13 emissive of blue light in the present instance, and in that this blue phosphor material is applied to the photosensitive material. Thereafter the screen is, of course, washed once again, in preparation for further processing.

The remaining portions of the screen structure illustrated in FIGURE l, namely the conductive layer 14 and the magnesium oxide strips 15, 4may then be deposited by any one of a number of conventional techniques such as those described in the copending application of Guy F. Barnett, Serial No. 367,181, ifiled July l0, 1953 and assigned to the assignee of the present invention, following which the screen structure may be incorporated in a complete cathode ray tube, also in any known conventional manner.

Considering now in detail the steps of applying the alkali salt or hydroxide solution in accordance with my invention, any one of the chemical substances which are properly designated as an alkali salt or as an alkali hydroxide may be used in solution for this purpose. However I have found that two of these substances, namely potassium silicate and lithium hydroxide, are particularly suitable for use in accordance with my invention, as they are especially effective in selectively reducing the retentivity for additional phosphor material of those portions of the previously formed screen structure in which there remain previously deposited phosphor material and exposed photosensitive material. At the same time these materials are particularly harmless to the remaining cornponents of the completed cathode ray tube. This is important because they are not removed by any of the subsequent operations and therefore are present within the finished cathode ray tube. In this connection it is noted that certain materials other than those here specied may adversely affect the operation of other portions of the cathode ray tube; in particular the cathode from which electrons are projected toward the screen structure are emitted is highly sensitive to contamination (called poisoning) by undesired foreign substances.

I have also found that, while the successful practicing of my method is critically dependent upon the concentration of alkali salt or hydroxide in the solution in which it is applied, the optimum concentration varies depending upon the particular substances which are used in the solution and also depending upon the particular phosphor materials to which this solution is applied. Thus with one particular phosphor material the proper concentration may have one value while with another phosphor material the best concentration of the same alkali salt or hydroxide as before may have a different value. Similarly different salts or hydroxides used with the same phosphor may have to be used in different concentrations for best results. For example, to prevent the adhesion of calcium magnesium silicate (a blue phosphor) to previously deposited zinc orthosilicate (a green phosphor), either a two percent aqueous solution of lithium hydroxide, or a two tenths of one percent aqueous solution of potassium silicate has been found suitable. Solutions of these same concentrations are also effective to prevent adhesion of zinc phosphate (a red phosphor) to calcium magnesium silicate. Optimum concentrations for other combinations of phosphor materials and alkali salts or hydroxides to be used for coating solutions can obviously be determined by simple tests well within the skill of a worker in the art.

In general it may be said, however, that the solution will always have to be a weak solution with the concentration of alkali salt or hydroxide ranging from approximately .1% to 5% by weight. This is because I have found that too high a concentration causes the formation of a layer of material on the exposed glass substrate which resists removal by subsequent treatment and which therefore prevents the application of phosphor materials to these exposed portions in subsequent steps, while too low a concentration results in the application of insutcient quantities of the material in solution to those portions on which it is desired to prevent the deposition of subsequent phosphor materials.

It will be noted that, in practicing my invention, it is not necessary to take any precautions to assure that the alkali salt or hydroxide applied to the exposed glass substrate is removed therefrom prior to the application of phosphor material in selected portions to this exposed glass substrate. This is because the alkali salts or hydroxides are highly soluble in the photosensitive materials which are deposited on these substrates before the deposition of additional phosphor material so that these alkali salts or hydroxides no longer coat these portions of the substrate prior to the deposition of additional phosphor material. On the other hand the subsequently deposited photosensitive material is incapable of removing the alkali salt or hydroxide from the previously deposited phosphor portions and the previously deposited and exposed photosensitive material because these salts and 7 Y hydroxides form intimate bonds with the phosphor and with the exposed photosensitive material. It is in this manner that the alkali salt and hydroxide deposited in accordance with my invention is believed to protect selectively only those portions on which phosphorus had previously been deposited from the deposition of additional phosphor while permitting the deposition of additional phosphor material on those portions of the screen structure on which no phosphor remains from a prior deposition.

As has previously been pointed out, Ithe application of an alkali salt or hydroxide, in accordance with the invention, need not be carried out as a separate step but may be carried out in the course of applying the rst and second phosphor layers, i.e., in the course of performing the operation represented in FIG. 2 by rectangle 18 and of performing the phosphor deposition portion of the operations represented in FIG. 2 by rectangle 21. In that case, the separate operations represented by rectangles 29 and 23 in FIG. 2 will, of course, be omitted. The optimum concentrations of alkali salt or hydroxide, when used in this alternative manner, appear to range somewhat lower than when applied in a separate operation. Thus or'lithium hydroxide, for example, the concentration in the phosphor suspension should range from Vapproximately 0.03 to 0.17 percent by weight.

It will be noted that my invention is applicable not only to the formation of the colored light emisssive phosphor strips of a cathode ray tube screen structure but also to the formation of certain other portions of cathode ray tube screen structure. For example, it has been proposed to apply, to the electron beam exposed surface of a conductive lm such as iilm 14 of the screen structure of FIGURE 1 a plurality of different colored light or ultraviolet light emissive phosphors which would serve to indicate, by their emission of light of different colors, the impingement of the electron beam upon different portions of the screen structure. In such an arrangement it is clearly also of importance to prevent the deposition of phosphor of one type in areas to be occupied exclusively by phosphor of another type. Clearly the invention is also applicable to the manufacture of such portions of a cathode ray tube screen structure. In view of the foregoing I desire the scope of my invention to be limited only by the appended claims.

I claim:

1. In the method of forming a cathode ray tube screen structure involving the steps of depositing a rst layer of unexposed photosensitive material on a substrate, exposing selected regions of said layer to illumination, de-

positing a rst phosphor over a portion of said layer including said exposed regions, removing from said substrate the unexposed regions of said layer and phosphor deposited thereon while leaving in place said exposed regions and phosphor deposited thereon, depositing a second layer of unexposed photosensitive material over said exposed regions and also over areas of said substrate from which said unexposed regions have been removed, and depositing a second phosphor over said second layer, the improvement which consists in applying to said exposed regions of said rst layer prior to said deposition of said second layer a weak aqueous solution of a material selected from the group consisting of potassium silicate and lithium hydroxide, thereby to prevent said second phosphor from adhering to said exposed regions of said iirst layer.

2. A method according to claim 1, wherein said unexposed photosensitive material comprises polyvinyl alcohol.

3. A method according to claim l, wherein said selected material has a concentration in said solution of between about 0.03 and about 5 percent by Weight.

4. A method according to claim l, wherein said solution of said selected material is applied subsequent to the deposition of said first phosphor on said iirst layer, and wherein said selected material has a concentration in said solution of between about 0.1 and about 5 percent by weight.

5. A method according to claim 1, wherein said selected material is lithium hydroxide.

6. A method according to claim 1, wherein said solution is an aqueous slurry comprising both said first phosphor and lithium hydroxide in a concentration of between about 0.03 and about 0.17 percent by weight, and wherein said tirst phosphor is deposited on said irst layer by applying said slurry thereto.

7. A method according to claim 1, wherein said solution consists essentially of water and potassium silicate.

References Cited in the file of this patent UNITED STATES PATENTS 2,376,437 Leverenz May 22, 1945 2,621,997 Benes Dec. 16, 1952 2,870,010 Sadowsky et al Ian. 20, 1959 2,929,708 Straw Mar. 22, 1960 2,959,483 Kaplan Nov. 8, 1960 2,992,107 Kaplan et al July 11, 1961 FOREIGN PATENTS 713,908 Great Britain Aug. 18, 1954 1,113,836 France Dec. 12, 1955 

1. IN METHOD OF FORMING A CATHODE RAY TUBE SCREEN STRUCTURE INVOLVING THE STEPS OF DESPOSITING A FIRST LAYER OF UNEXPOSED PHOTOSENSTIVE MATERIAL ON A SUBSTRATE, EXPOSITING SELECTED REGIONS OF SAID TO ILLUMINATION, DEPOSITION A FIRST PHOSPHOR OVER A PORTION OF SAID LAYER INCULDING SAID EXPOSED REGIONS, REMOVING FROM SAID SUBSTRATE THE UNEXPOSED REGIONS OF SAID LAYER AND PHOSPHOR DEPOSITED THEREON WHILE LEAVING IN PLACE SAID EXPOSED REGIONS AND PHOSPHOR DEPOSITED THEREON, DEPOSITION A SECOND LAYER OF UNEXPOSED PHOTOSENSTIVE MATERIAL OVER SAID EXPOSED REGIONS AND ALSO OVER AREAS OF SAID SUBSTRATE FRON WHICH SAID UNEPOSED REGIONS HAVE BEEN REMOVED, AND DEPOSITING A SECOND PHOSPHOR OVER SAID SECOND LAYER, THE IMPROVEMENT WHCIH CONSISTS IN APPLYING TO SAID EXPOSED REGIONS OF SAID FIRST LAYER PRIOR TO SAID DEPOSITION OF SAID SECOND LAYER A WEAK AQUEOUS SOLUTION OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF POTASSIUM SILICATE AND LITHIUM HYDROXIDE, THEREBY TO PREVENT SAID SECOND PHOSPHOR FROM ADHERING TO SAID EXPOSED REGIONS OF SAID FIRST LAYER. 